This application is related to co-pending application Ser. No. 168,097, filed Mar. 14, 1988, in the names of E. Clark and D. R. Amos, entitled xe2x80x9cRepair Welding Low Alloy Turbine Componentsxe2x80x9d, which is assigned to the assignee of this application and which is hereby incorporated by reference.
This application is also related to co-pending application Ser. No. 092,851, filed Aug. 24, 1987, in the names of R. T. Ward and J. M. Butler, entitled xe2x80x9cRepair of High-Pressure Turbine Rotors By Ring Weldingxe2x80x9d, which is assigned to the assignee of this application and which is herein incorporated by reference.
This application is also related to application Ser. No. 763,744, filed Aug. 8, 1985, in the names of R. E. Clark and D. R. Amos, entitled xe2x80x9cMethod for Repairing A Steam Turbine Or Generator Rotorxe2x80x9d, now U.S. Pat. No. 4,633,544, issued Jan. 6, 1987, which is assigned to the assignee of this application and which is herein incorporated by reference.
This application is also related to co-pending application Ser. No. 727,175, filed Apr. 25, 1985, in the names of R. E. Clark, D. R. Amos, and L. M. Friedman, entitled xe2x80x9cHigh Strength, High Toughness Welding for Steam Turbine Rotor Repairxe2x80x9d, which is assigned to the assignee of this application and which is hereby incorporated by reference.
1. Field of the Invention
This invention relates to repair procedures for worn or damaged surfaces of turbine components, and in particular, to welding techniques for building up these worn surfaces with sound metal deposits.
2. Background of the Invention
Steam turbine components made of Cr-Mo-V alloys, such as rotors and discs, provide optimum high-temperature fatigue and creep properties, but are considered difficult to weld. However, since the down time associated with replacement of these often worn, eroded, or cracked components can cost electric utilities hundreds of thousands of dollars per day, many procedures have been attempted to repair them.
One such repair procedure consists of welding an individual piece of forged steel to a worn rotor or disc. However, when this type of repair is made on a single rotor blade groove fastening, herein referred to as a xe2x80x9csteeplexe2x80x9d, welder accessibility is very limited. Accordingly, a weld repair conducted with very limited accessibility can result in unacceptable, non-destructive examination quality due to the formation of porosity cracks and slag inclusions.
It is also known to make rotor repairs by submerged arc welding after a low volume welded seam is made between a turbine component and a forged replacement section. See Kuhnen, U.S. Pat. Nos. 4,213,025 and 4,219,717, which are herein incorporated by reference. In such a procedure, a ring forging is welded to a worn disc or rotor or a completely new rotor forging is welded to replace the entire end of the rotor. See Clark et al. U.S. Pat. No. 4,633,554, disclosing a narrow gap weld root pass followed by a gas metal arc build-up for this purpose. The lower tensile and fatigue properties obtained by employing this process, however, are generally insufficient for use in high stress rotor steeple areas.
Submerged arc welding alone has also been used for build-up repairs of rotor areas involving a wide or deep groove, where a cracked defect is not oriented longitudinally along the radius of the rotor. The main advantage of building up with submerged arc welding is that this procedure has a very high deposition rate, typically about 15 pounds of weld metal per hour. The higher deposition rate is important since many of the service rotor weld repairs are made during turbine outages, thus, time is extremely important. However, this process requires a pre-heat, produces a relatively large grain size with inferior metallurgical properties. Typically, these submerged arc weldments on low pressure rotors have a yield strength of about 85 to 100 Ksi (586 to 689 MPa) and a room temperature Charpy toughness of about 100 to 120 ft-lbs (136 to 163 J). It is also understood that submerged arc weldments are often rejected due to poor ultrasonic quality, which often reveals slag inclusions and porosity in the weld metal. Moreover, serious creep-rupture and notch-sensitivity problems have been encountered with high-pressure Cr-Mo-V rotor repair welds manufactured from submerged arc weldments. Thus, the submerged arc process is generally unacceptable for weld repairs of Cr-Mo-V rotor steeples having small, high-stress concentration radii.
Gas metal arc procedures have also been employed for repairing rotors and discs. This welding procedure deposits about 8 lbs of weld metal per hour, typically having slightly better properties than weldments produced by the submerged arc process. For Cr-Mo-V rotor repair welding, the gas metal arc weldments of steel turbine components generally have a yield strength of about 85 to 100 ksi (586 to 689 MPa), and a room temperature Charpy toughness of about 110 to 130 ft-lbs (150 to 177 J). The gas metal arc welding process associated with welding these alloys, however, is often associated with arc-blow (magnetic) process limitations which can limit the use of this process.
Recently, emphasis has been placed on the use Of gas tungsten arc welding processes (GTAW) for making repairs on Ni-Mo-V and Ni-Cr-Mo-V low pressure rotor components. See R. E. Clark, et al. xe2x80x9cExperiences with Weld Repair of Low Pressure Steam Turbine Rotorsxe2x80x9d, 47th American Power Conference, Apr. 22-24, 1985, Chicago, Ill., printed by Westinghouse Electric Corporation, Power Generation, Orlando, Fla., herein incorporated by reference. Gas tungsten arc welding has been employed for repairing individual rotor attachment grooves, cosmetic, or shallow groove repairs to correct minor surface defects. It has also been used to allow multiple build-ups of plate attachment groove locations, i.e., for a 360xc2x0 application, and cladding to restore worn-away material. Gas tungsten arc welding offers relatively high ultrasonic quality, requires no pre-heat, and produces weldments having tensile and impact properties which exceed rotor material specification requirements. Low allow steel weldments produced by this process typically have a yield strength of about 90 to 115 ksi (621 to 793 MPa), and a room temperature. Charpy toughness of about 160 to 210 ft-lbs (218 to 286 J). In addition, this welding procedure produces the finest microstructural grain size of any of the above-mentioned processes.
The selection of a weld method depends on factors such as distortion, non-destructive testing acceptance limits, and mechanical property response to the post-weld heat treatment. Each area of a turbine rotor is unique, and experiences a different service duty. The absence of weld and heat affected zone cracking as well as the minimization of defects, can only be accomplished by carefully controlling a number of welding variables. For the gas tungsten arc welding process, some of these variables include amperage, alloy selection, joint geometries and travel rate. The parameters selected should be accommodating to automatic welding processes to obtain a uniform quality which is reproducible from weld to weld. These parameters must also produce superior welding characteristics such as freedom from porosity, cracking, and slag entrapment, while being accommodating to all possible repairs on rotors and discs. Finally, the alloy and welding parameters selected must produce a weld comparable to the properties of the base metal.
Accordingly, a need exists for a welding procedure that maximizes the metallurgical properties of the repaired area of turbine components. There is also a need for a welding procedure that minimize the heat affected zone and eliminates weld-related cracking.
Improved turbine systems including more failure resistant rotors and novel methods for repairing worn surfaces of steam turbines, especially high pressure turbine rotors are disclosed. The methods include welding procedures and heat treatments that minimize weld stresses and cracking. The procedures employed substantially reduce the risk of failure in ferrous Cr-Mo-V base metals of high-pressure, high temperature rotors and discs commonly found in steam turbines. This invention presents an improvement over welding forged fastenings to rotors, since welder accessibility and weldment integrity are improved. These features are particularly important with respect to high pressure, (HP), turbine components, such as rotors, which have been known to operate at pressures over 2400 psi and temperatures over 1000xc2x0 F.
The invention includes depositing a first layer of weld metal on a worn surface of a turbine component and then depositing a second layer of weld metal over the first layer, using an higher application temperature, for tempering at least a portion of the xe2x80x9cheat-affected zonexe2x80x9d (HAZ) created in the base metal by the depositing of the first layer. As used herein, the term xe2x80x9cheat affected zonexe2x80x9d refers to the area of base metal immediately adjacent to the fusion zone of the weldment.
Accordingly, improved welding methods are disclosed for overcoming the occurrence of metallurgical structural problems within the heat-affected zone. The additional heat generated by the deposition of the second layer of weld metal produces an immediate heat treatment of the heat-affected zone, whereby coarse grains of the base metal are recrystallized and tempered. It is understood that when these course grains are reformulated into a finer grain structure, stress-relief cracking in the vicinity of the weld repair can be minimized.
The methods employed by this invention also avoid the over-tempering, or softening, of the base metal created by the heat of welding the first layer of weld metal. This loss in strength occurs, to a greater extent, when a stress transverse to the weld is applied, for example, high and low cycle fatigue, tensile, or creep-to-rupture. The proper control of the initial layers of weldment can significantly reduce the failure in the heat-affected zone and prevent the loss of strength in this zone below the levels of the unaffected base metal.
Further improvements disclosed by this invention include the use of bead sequencing for minimizing heat input into the base metal. Run-off tabs are also taught for minimizing weld defects created by starting and stopping the arc. In addition, a weld trail-shield is disclosed for minimizing carbon losses in the base metal which could result in lower tensile properties. Finally, parameters such as preheat-interpass temperatures, shield gas-type and flow rates, current, tungsten size and weld speed are also disclosed for achieving a higher quality weld. Procedures for single xe2x80x9csteeplexe2x80x9d repairs and for 360xc2x0 rotor repairs are also separately disclosed.
It is, therefore, an object of this invention to provide repair welding procedures compatible with high pressure, chromium-containing rotors and other components currently in service.
It is another object of this invention to provide welding procedures, alloys, and heat treatments which provide improve notch sensitivity and increased creep ductility to repaired or new turbine components.
It is still another object of this invention to provide a repaired turbine rotor for use in high pressure service which is relatively free of weld porosity, lack of fusion and cracking resulting from the welding process.
With these and other objects in view, which will become apparent to one skilled in the art as the description proceeds, this invention resides in the novel construction, combination, arrangement of parts and methods substantially as hereinafter described and more particularly defined by the attached claims.